1. Field of the Invention
The present invention relates to the signal process of FM-CW (frequency modulation-continuous wave) type radar apparatus, and in particular it relates to a crossover detection method for reducing the number of crossover (measurement errors), a radar apparatus thereof and a crossover detection program thereof.
2. Description of the Related Art
An FM-CW radar apparatus that measures the relative velocity and distance of a target object. This FM-CW radar apparatus can measure the relative velocity and distance of a vehicle that is running ahead using a simple signal processing circuit, containing an easily configurable transmitter-receiver. Therefore, the FM-CW radar apparatus is used as a radar apparatus for preventing vehicles from colliding with one another.
FIG. 1 shows a usage example of such a radar apparatus.
In FIG. 1, a radar apparatus 1101 mounted on a vehicle 1102 can measure the distance with and relative velocity of vehicles 1103 and 1104 that are running ahead of it, and based on the information, the vehicle 1102 can follow one of the vehicles (for example, vehicle 1104) or apply the brakes to prevent a collision.
FIG. 2 shows the summary of the FM-CW radar apparatus.
A radio transmitter 1201 transmits radio waves, and a radio receiver 1203 receives the radio waves reflected by a target 1202 being measured. The propagation time from the transmission until reception of each of the radio waves and its Doppler shift are measured by processing a beat signal obtained by mixing the received signal with its original transmitting signal. From this data, distance (D) and the relative velocity are calculated.
For example, assume that an FM-modulated wave, modulated by a triangle wave of several hundred Hertz, is transmitted and the signal reflected by the target 1202 is received. Then, the received signal is FM-detected using an FM-modulated wave as a local signal. Deviation (beat) occurs between the reflected wave from the target 1202 and its original transmitting signal in accordance with the distance between the FM-CW radar apparatus and the target 1202 or in accordance with the Doppler shift due to the relative velocity. Therefore, the distance and relative velocity of the target 1202 can be calculated using this frequency deviation.
Since in the FM-CW radar apparatus, a triangle wave is most often used as a modulation signal, the case where a triangle wave is used as a modulation signal is described below. However, a radio wave other than a triangle wave, such as a sawtooth wave, a trapezoidal wave or the like, can also be used as a modulation wave.
FIG. 3 shows the principle of the FM-CW radar.
FIG. 3 shows a graph using time t and frequency f as its horizontal and vertical axes, respectively. The triangle wave represented by a solid line indicates the frequency of a transmitting wave. The triangle wave represented by a dotted line indicates the frequency of a radio wave received from a fixed object located at a distance D from the radar apparatus. Frequency deviation occurs during the propagation time between the two waves. In this case, measuring the frequency difference between a transmitting wave and its received wave, which is proportional to the propagation time, is simpler than directly measuring the respective propagation times.
FIG. 4 shows the principle of the FM-CW radar apparatus in the case where the relative velocity of a target is 0.
The transmitting wave is a triangle wave, and its frequency changes are shown by a solid line in FIG. 4A. As shown in FIG. 4A, a frequency difference fr signal (beat signal) is generated from the difference in frequency between a transmitting wave and its received wave. Then, the beat signal is sampled in each of the upward and downward ranges of the triangle wave, and power can be calculated by applying Fourier transformation to the sampled beat signal. Since the resulting power peak appears due to the frequency difference fr (FIG. 4C), the peak is detected and a distance D is calculated based on fr.
FIG. 5 shows the principle of the FM-CW radar apparatus in the case where the relative velocity of a target is V.
It is seen from FIG. 5 that in this case, a deviation of the propagation time and a deviation fd of Doppler shift occur.
A beat signal is sampled in each of the upward and downward ranges of a triangle wave and power is calculated by applying Fourier transformation to each sampled beat signal. Then, a frequency peak fup can be calculated by subtracting Doppler shift deviation fd from frequency difference fr due to propagation time in the upward range, and a frequency peak fdown can be calculated by adding Doppler shift deviation fd to the frequency difference fr due to propagation time in the downward range. A frequency difference 2xc3x97fr due to the propagation time can be calculated by adding fup to fdown, and the Doppler shift deviation 2xc3x97fd can be calculated by subtracting fdown from fup. 2xc3x97fr and 2xc3x97fd can be converted into distance and relative velocity, respectively, by multiplying constants.
However, there is often measurement error in a conventional FM-CW radar apparatus.
FIGS. 6 and 7 show the problems of the conventional FM-CW radar apparatus.
The Doppler shift shown in FIG. 6B is larger that shown in FIG. 6A. Although the difference between fup1 and fup2 of a beat signal can be seen from FIG. 6, the difference is positive or negative and there is no difference in power between them as a result of Fourier transformation. In other words, if the difference of Doppler shift is determined based on only the result of power, a measurement error occurs. If the distance is pretty short and the relative velocity is pretty fast, there is often such a phenomenon.
A method of comparing phases is also conceivable. However, since there is no reference phase, such comparison is practically impossible. Such a measurement error is called xe2x80x9ccrossoverxe2x80x9d.
The object of the present invention is to provide a crossover detection method for reducing the number of crossover (measurement errors) in a radar apparatus, a radar apparatus thereof and a crossover detection program thereof in order to solve the problems described above.
The present invention is a crossover detection method that is executed in a radar apparatus in order to calculate the distance and relative velocity of a target, using a beat signal calculated by mixing a received signal with its original transmitting signal. The method comprises calculating distance/relative velocity information indicating the distance/relative velocity, respectively, at a multitude of clock times; calculating predicted distance/relative velocity information indicating the distance/relative velocity, respectively, of a target after a prescribed time elapses from the calculated distance/relative velocity information, respectively; calculating the predicted distance/relative velocity errors by calculating the difference between the calculated distance information and the calculated predicted distance information, and between the calculated relative velocity information and the calculated predicted relative velocity information, respectively; calculating the degree of similarity information based on a predetermined average predicted by using distance/relative velocity errors and the calculated predicted distance/relative velocity errors, respectively; and determining whether or not there is crossover, based on the calculated degree of similarity information.
According to the present invention, the number of crossover (measurement errors) in a radar apparatus, in particular a FM-CW type radar apparatus can be reduced.